My configuration boils down to Linux VMs running Ubuntu 18.04 LTS with Horizon Agent 7.12.0 being viewed remotely by the Horizon Client 5.4.2.
When the Horizon Client connects, the gdm3 login screen displays, but only updates when the mouse is used to open a menu. Simply typing in to the username / password field causes no obvious graphical changes. But, if you right-click on the input field or click on the power drop-down menu, the username / password field reflects the text that was typed. If the accessibility menu is used to enable 'zoom', the field updates as expected. So, at least there is a work-a-round.
Everything else about the sessions are nominal. Once a user has logged in and started either a Gnome 3 or a MATE session, everything behaves and redraws as expected. This only impacts the gdm3 login screen.
Has anyone else experienced this behavior?